1. Field of the Invention
The present invention relates to a method for encapsulating a molded member of ceramic, particularly a silicon ceramic material for high-temperature isostatic pressing.
During high-temperature isostatic pressing (HIP), porous molded members of silicon ceramic materials are compressed under the concurrent application of high pressure acting on all sides thereof and high temperature. High-temperature isostatic pressing facilitates improvement in the mechanical strength at elevated temperature, as well as the resistance to oxidation through a reduction in the porosity of the molded members. The pressures which are applied during high-temperature isostatic pressing (HIP) are in a magnitude of 3000 bar, with temperature being about 1750.degree. C. Gas is usually employed as the pressure-transmissive medium. In order to prevent this gas from penetrating into the pore structure of the molded member, the latter must be encapsulated in a pressure-tight manner.
2. Discussion of the Prior Art
Heretofore, this encapsulation has been effected in a manner wherein the molded member is enclosed in an evacuated glass container, whereby the glass container assumes a viscous state during the high-temperature isostatic pressing and adheres to the molded ceramic member in the form of a glass coating.
In this known method for the encapsulating of the molded ceramic member, there is present the danger of the glass coating penetrating into the pore structure of the molded member at too low a viscosity of the glass container melting during the high-temperature isostatic pressing whereby, depending upon circumstances, this will impair the strength of the molded member in the surface region thereof. Furthermore, due to excessive penetration of the highly-fluid glass into the pore structure of the molded member, such an intimate bond can be produced between the glass coating and the molded member that it may be unavoidable to prevent damage being sustained by molded ceramic members, particularly those with complex, thin-walled configurations, during removal of the glass envelope after the high-temperature isostatic pressing. On the other hand, there is encountered the danger that at too high a viscosity of the glass container melted during high-temperature isostatic pressing and the resultant glass coating adhering to the molded ceramic member, there will occur deformations of the molded ceramic member, which are absolutely to be avoided.